Transistors are often used in switching circuits to perform a current switching function in order to selectively supply current to loads in response to control signals. In such switching circuits, the switching transistors typically having a current path coupled between a load and a bias network that includes a constant current source. The control electrode of each switching transistor receives a control signal to selectively turn that switching transistor on to switch a current determined by the bias network to the associated load.
In presently available current switches in which the switching transistors are bipolar transistors, the current switched to a load is dependent upon the switching transistor's current gain .beta.. Since .beta. varies significantly as a result of process variations, the value of the current switched to the load is also subject to process variations. The complete elimination of all process variations is extremely difficult. As a result, .beta. can typically be guaranteed only to be within a fairly broad range thus making it impossible to predict with a high degree of accuracy what the value of the switched current will actually be.
This inability to switch a known, very accurate current is undesirable in a wide variety of devices, such as oscillators using current ramping techniques, accurate clock duty cycle control circuits, and transconductance amplifiers, that require accurate current switching capability.
Accordingly, a need exists for a current switch having bipolar switching transistors that has reduced sensitivity to process variations and can switch a predetermined current with a high degree of accuracy.